* Cameron wants Britain's EU ties to change "very radically"
* Says he is convinced EU will have to change its treaties
* Anti-EU party poses threat to Cameron at 2015 election
* Cameron: UK may leave European Convention on Human Rights
By Peter Griffiths
LONDON, Sept 29 (Reuters) - Prime Minister David Cameron ratcheted up the rhetoric against Europe on Sunday by demanding a radical change to Britain's ties with the European Union and offering the prospect of an exit from the continent's principal treaty on human rights.
Europe poses one of the biggest obstacles to Cameron's re-election in 2015 and he is under pressure from Conservative lawmakers to stem the loss of support to the UK Independence Party (UKIP), which calls for an immediate withdrawal from the EU.
Cameron said the EU will have to renegotiate the treaties on which it was founded, an idea rejected by some EU members as being too complicated and time-consuming.
"My goal is to renegotiate our relationship with Europe, very radically," Cameron told the BBC at the start of his Conservative Party's conference in Manchester, northern England. "We need a treaty renegotiation, I am convinced one has to happen."
Cameron said in January that he would negotiate Britain's EU relationship before holding an in/out referendum by the end of 2017, provided he wins the next election in 2015.
That promise was seen as an attempt to mollify anti-EU Conservatives and mount a stronger challenge to UKIP.
Some EU members have warned Britain that they will resist attempts to reclaim powers unilaterally. Business leaders say talk of Britain leaving its biggest trading partner could undermine confidence and investment.
German Chancellor Angela Merkel, who was re-elected with a strong mandate this month, has started talking about powers flowing back to member states from Brussels, but "she means something different from the British" and is still committed to a more integrated EU, an aide told Reuters.
With an election less than two years away, Cameron said the Conservatives were the only party guaranteeing voters a straight in/out referendum by a specific date.
The opposition Labour Party, leading in opinion polls, says an in/out referendum is currently not in the national interest. The Liberal Democrats, junior member of Cameron's coalition government, decided earlier this month to back an in/out referendum, but only if there is a big change to the treaties that govern the EU.
Voters don't like the idea of an "ever closer union", one of the EU's guiding principles, and think the alliance has become too bureaucratic and anti-competitive, Cameron said. Polls show more Britons want to leave the EU than remain a member, although many are undecided.
Cameron said he would consider abolishing the Human Rights Act, which since 2000 has made the European Convention on Human Rights (ECHR) enforceable in Britain's courts.
Although the ECHR is not part of the EU system, British eurosceptics often drag it into the Europe debate, using it as an example of external interference in Britain's sovereignty.
The case of radical cleric Abu Qatada - who resisted British attempts to send him to Jordan to face terrorism charges for more than a decade - angered many voters and exposed successive governments to the accusation that they do not control Britain's borders.
Asked if Britain should leave the Convention, Cameron said: "It may be that that is where we end up."
The Convention was drafted by the Council of Europe in 1950 and was ratified by a British Conservative government. It is enforced by the European Court of Human Rights. Both pre-date the establishment of the European Economic Community by several years and have no organic link with the European Union or the European Court of Justice.